Pipeline contractors are inspect materials and quality of work of pipeline construction to conform to specific specifications. Pipeline contractors also make sure the depth and alignment of trenches and examines bottom of trench for rocks or other hard objects that might damage pipe coatings. Pipeline contractors also examine surface and x-ray films of welded joints for detection of external and internal welding defects. Pipeline contractors also coat and wrap pipe or pipe joints. Pipeline contractors can be dry utility pipeline contractors or wet utility pipeline contractors. Dry utility pipeline contractors cover projects related to natural gas, electrical, and communications while wet utility pipeline contractors deal with the transfer of water, petroleum products, and installation of sewer pipelines and other underground pipes that transport oil to refineries. Pipeline systems are categorized as residential, commercial, industrial, institutional, and public works. Pipeline contractors are generally hired by the General Contractors.